Field
The present invention relates to wind turbine towers. In particular, the present invention relates to self-supporting wind turbine towers.
Description of the Related Art
Conventional towers for large wind turbines are generally either tubular steel towers, lattice towers, or concrete towers. Most are tubular steel towers which are formed from several 20-30 meter sections bolted together in situ. The tower is then fixed either to a foundation made of concrete using anchor bolts (onshore), to a mono pile, gravity or tripod foundation (off-shore, shallow), or to a floating foundation (off-shore, deep water).
When designing a wind turbine tower, it is necessary to consider its natural frequency in relation to the passing frequencies of the rotor. These passing frequencies are defined as the frequency of one complete revolution of the rotor and the frequency of any blade passing the tower, i.e. the rotating frequency of the complete rotor, divided by the number of blades. If the natural frequency of the tower is in the region of either of the passing frequencies of the rotor, resonance may occur causing the magnitude of the vibrations felt by the turbine to increase.
The natural frequency of a tower decreases in proportion to its length squared. Thus, the natural frequencies of longer towers are typically lower than those of shorter towers. As tower lengths have increased in order to accommodate larger blades, the natural frequency of a conventional tower may lie close to the passing frequencies of the rotor. Consequently, longer towers can be susceptible to resonance which may result in damage to the turbine components or tower foundations.
This problem is compounded by the fact that turbines are usually designed to work within a certain range of rotational speeds. Thus, the passing frequencies to be avoided are generally frequency ranges, rather than narrow bands or fixed amounts.
US 2009/0266004 discloses a wind turbine tower formed from a carbon fibre composite. The tower is fabricated by preparing a flexible textile preform offsite, transporting the preform to the assembly location, placing the preform over a mandrel and laminating the preform with a resin to form the composite shell. As the natural frequency of a tower is proportional to the square root of the specific stiffness of the material from which the tower is made (the specific stiffness being an inherent property of the material and which is defined as E/ρ, where E is the Young's modulus and ρ is the density), the use of a stiff carbon fibre composite results in a tower with an increased natural frequency. Consequently, the tower is less prone to excitation modes under varying load conditions. However, carbon fibre composite towers are significantly more expensive to manufacture than equivalent steel towers.
US 2011/0138707 discloses a wind turbine tower having a concrete lower portion and a steel upper portion and teaches that this arrangement allows the height of a conventional steel tower to be increased without a corresponding increase in the difficulty of tower construction and transport.
However, when constructed, such a tower will have a lower natural frequency in comparison to a smaller, conventional steel tower. Consequently, the natural frequently may lie close to the passing frequencies of the rotor, increasing the risk of resonance and the associated damage to the turbine components or tower foundations.